1. Field of the Invention
The present invention relates to a passenger protecting apparatus such as air bag or seat belt pretensioner which detects the collision of a vehicle, and more particularly to the diagnosis of the passenger protecting apparatus.
2. Prior Art
FIG. 6 shows a prior art air bag actuating apparatus, one type of passenger protecting apparatus disclosed in Japanese Patent Preliminary Publication No. 1-102372. In FIG. 6, an on-vehicle battery 1 supplies power to various circuits. An impact switch 2 is a mechanical switch whose contacts are opened and closed by shocks exerted on the vehicle. A squib 3 is an electric resistor that serves as a heater to ignite an air bag. Parallel-connected firing transistors 4 and 40 are inserted in series with the squib 3. The transistors 4 and 40 operate as firing switches which are switched on and off by a control signal supplied thereto.
Monitor resistors 21, 22, and 23 detects the cut off and short-circuit of the firing current path, the resistor 21 being connected in parallel with the impact switch 2, the resistor 22 with the firing transistor 4, and the resistor 23 with the firing transistor 40. A piezoelectric acceleration sensor 24 and has electrodes 24a-24d. An amplifier 25 amplifies the output of the acceleration sensor 24. A first and a second calculating circuit perform calculations based on the output of the amplifier 25 to determine whether the vehicle collided or not. A first and a second drive circuit are connected with the outputs of the first and second calculation circuits 26 and 27 and output the control signals for turning the transistors 4 and 40 on and off. A diagnosis circuit 30 is connected with the output of the amplifier 25 and performs diagnostic operation to determine whether the acceleration sensor 24 is malfunctioning. A pulse generating circuit 31 is connected with the diagnosis circuit 30 and outputs pulses for diagnosis to the electrodes 24b and 24d of the acceleration sensor 24. An alarm lamp 12 comes on if the acceleration sensor 24 is not troubled and does not come on if troubled.
The operation of the prior art apparatus will now be described below. The acceleration sensor outputs a voltage proportional to the deceleration through the electrodes 24a and 24c when the vehicle is decelerated. The voltage is amplified by the amplifier 25 and is then directed to the first and second calculation circuits 26 and 27.
When the deceleration of the vehicle is less than a predetermined value, the output of the amplifier 25 is small and therefore the first and second calculation circuits outputs no signal. The transistors 4 and 40 remain turned off so that the squib 3 generates no heat. Thus, the air bag is not actuated.
When the vehicle collides and the deceleration thereof exceeds a predetermined value, the output of the amplifier 25 is also large causing the first and second calculation circuits to output signals. The signals from the calculation circuits cause the first and second drive circuits 28 and 29 to turn on the transistors 4 and 40. At this time, the impact switch 2 is also closed due to the excessive impact. Thus, the squib 3 is energized to generate heat so that powder is exploded to instantly inflate the air bag, thereby holding the passenger at the passenger seat to protect the passenger.
The pulse generator 31 outputs diagnosis pulses when performing diagnostic operation to determine whether the acceleration sensor 24 functions normally. The diagnosis pulses are supplied to the electrodes 24b and 24d of the sensor 2. If the acceleration sensor 24 is operating normally, the piezoelectric elements operates on the diagnosis pulses and voltage in accordance with the diagnosis pulses appearing across the electrodes 24a and 24c. The voltage is then amplified by the amplifier 25 and is then supplied to the diagnosis circuit 30. The diagnosis circuit 30 makes a check to determine whether the voltage across the electrodes 24a and 24c falls within a predetermined range. If the voltage is within the determined range, the diagnosis circuit 30 causes the alarm lamp 12 to glow indicating that the acceleration sensor 24 is normal.
When the acceleration sensor 24 is in an abnormal condition, either the acceleration sensor 24 outputs no signal or the voltage is not within the predetermined range. The alarm lamp 12 will not glow.
With the aforementioned prior art, the diagnosis operation is carried out only for the sensor. The circuits such as the signal processing circuit and the firing switch elements are not checked at all.